One way to hold this word: a space that has been left open, with the shape of what belongs there still visible. A line ruled on a form, a seat in a row, a box on a shelf. The room is not an accident — something was meant to go in it and has not arrived yet. Put that blank on a staff list and it is a job with no person; put it on an order book and it is goods that have not gone out.
From the word’s history
From un- plus filled. The prefix un- goes back through Middle English and Old English un- to Proto-Germanic, always meaning not. So the word can be read straight through: not filled.
unfilled 是 un- 加上 filled 造出來的。un- 一路往回追是中古英語 un-、古英語 un-，再到原始日耳曼語 *un-，意思從頭到尾都是「不、沒有」。所以這個字可以整個拆開來讀：沒有被填。職缺和訂單那兩個用法，就是把這個空格畫在不同的表上。
Wiktionary via kaikki snapshot 3f6f69a190036ae4, unfilled etymology 1: From un- + filled; un- from Middle English un-, Old English un-, Proto-Germanic *un-
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